Modern optical communication links utilize optical amplifiers to amplify wavelength division multiplexed (WDM) signal channels as they are transmitted through a link. The most common type of commercial optical amplifiers are Erbium doped fiber amplifiers (EDFAs). EDFAs belong to a broader class of amplifiers known as rare earth doped fiber amplifiers. EDFAs are self contained units, typically placed at intervals of 60-150 km along a fiber link. As with most optical amplifiers, EDFAs include a gain medium, specifically an Erbium doped fiber (EDF), and at least one pump laser. The gain medium serves to transfer energy from pump lasers within the EDFA to the optical signal channels as they pass through the EDFA, thus amplifying the signal channels. In most communications systems, the signal channels are located in the so called C-Band (1528-1565 nm). However, they may also be located in the L-Band (1570-1605 nm), and sometimes in adjacent wavelength bands
Since an optical amplifier also includes pump lasers, it is necessary to supply it with electrical power in order for it to function. In cases where the amplifier needs to be placed in remote or inaccessible locations, this can become prohibitively expensive. For example, in a sub-sea communication link, the amplifier may need to be placed in a portion of a link which is submerged, requiring a very expensive submarine cable capable of transmitting electrical power from a landing station. Such an expensive cable may not be feasible or practical for all applications.
To address these and similar cases, Remote Optically Pumped Amplifiers (ROPAs) were developed, as described for example in U.S. Pat. Nos. 5,321,707, 7,508,575 and 7,665,909, and in U.S. patent application Ser. No. 12,202,100. In these amplifiers, the pump lasers are located along the communication link separately from the gain medium and utilize an optical fiber (in many cases the transmission fiber itself) to transfer the pump energy to the gain medium. Thus, the pump lasers may be placed at a location where it is easier to supply electrical energy, while the gain medium may be placed at another location which provides better Optical Signal to Noise Ratio (OSNR), and thus better overall link performance.
In general, the pump power delivered into the transmission fiber by the pump lasers can be 30 dBm and higher (see description of FIG. 1 below). Such high optical power propagating along the transmission fiber can pose a potential safety hazard to persons coming into contact with the system. Particularly, if the pump lasers are operated while a connector along the transmission fiber is open, or when there is a break or cut in the fiber, the pump energy may escape and cause harm to human eyes or skin, as well as material damage to the system. As used herein, the term “open fiber” refers to the state where there is an open connector or break or cut within the transmission fiber, or any other situation that could cause significant leakage of pump power from the fiber, thus posing danger to human eyes or skin coming in contact with the leaked power. The term “opening” is used to refer to the point along the fiber where the leakage of power occurs. Clearly, there is a need to immediately detect any such open fiber and shut down the pump lasers (or reduce their power to a safe level) within a time span short enough to avoid harm to human eyes (henceforth referred to as “eye-safe time”). Exemplarily, International Standard IEC 60825-2, “Safety of Laser Products—Part 2: Safety of optical fiber communication systems”, may be viewed for a discussion of various aspects related to safety of laser products within fiber optic communication systems.
In other words, there is a need for an automatic shutdown mechanism in case of a safety hazard caused by an open fiber. The automatic shutdown mechanism should on one hand be as fail-safe as possible, and on another hand not be activated mistakenly by events that do not pose potential safety hazards. Another desired feature is that the shutdown mechanism should be an integrated feature of the ROPA system, to further enhance safety and to avoid dependence on other parts of the communication system.
These requirements have been partly recognized in the past, and a number of methods and systems have been disclosed to address the problem. For example, U.S. Pat. No. 6,423,963 discloses monitoring of an optical supervisory channel (OSC) existing in many commercial communication systems to detect an open fiber. One disadvantage of using an OSC for monitoring is that it is not present in all systems, and in any case it involves relying on a feature external to the ROPA system. Another disadvantage of using the OSC is that it constitutes a single point of failure in the system, i.e. failure of this channel will lead to shut-down of the ROPA, which in turn will shut-down the entire system. Furthermore, the OSC may be located in a wavelength band which is not amplified by the ROPA system, in which case it cannot be used. For example, in many cases the OSC is located at 1510 nm, which cannot be effectively amplified by an EDF based gain unit.
Another mechanism, disclosed in U.S. Pat. No. 6,423,963, is related to the monitoring of pump energy back-reflection, which can be used to detect certain types of open connectors but not fiber breaks or cuts. For example, opening a polished connector (PC) within a certain distance of the ROPA pump unit will cause a detectable increase in pump energy back-reflection. A main disadvantage of this solution is that it is not sensitive to certain types of open fibers (e.g. fiber breaks or cuts or open angle polished connectors (APC) connectors).
U.S. Pat. No. 7,031,049, and U.S. Pat. Nos. 7,116,471 and 7,283,292 (the latter two assigned to the present assignee) disclose the use of ASE noise created by the Raman scattering effect as pump energy propagates along a transmission fiber (sometimes referred to as amplified spontaneous scattering—ASS), in order to detect an open fiber. However, in ROPA systems, the ASE noise created by the ROPA gain unit is typically much higher than the ASE noise created by the Raman effect within the transmission fiber, such that detection methods relying only on Raman generated ASE are unusable in ROPA systems.
Thus, there is a need for methods and apparatus for detecting an opening in an optical transmission fiber which is indicative of a laser (or “safety”) hazard for ROPA systems which does not suffer from the shortcomings described above. There is a further need for an automatic shutdown mechanism in case of a safety hazard caused by an open fiber in such systems. Specifically, the mechanism should be self-contained within the ROPA system, and not be dependent on other features of the communication system of which the ROPA is part. Furthermore, it should be sensitive to all types of open fibers, and it should provide real-time continuous detection of an open fiber during operation of the ROPA system.